A pressure cap comprises a closing part associable with an opening of a container, engaging means associable with a neck of said container, said engaging means being provided with retaining means for engaging in a projection of said container, a line of intended opening in said engaging means interposed between a side body of said engaging means and said closing part, said line of intended opening being defined by fracturable means, said line of intended opening and said closing part being positioned in relation to one another so that said fracturable means breaks along said line of intended opening when said closing part is induced to detach itself from said side body.